Effect of powder feedstock modification and building orientation on the static and fatigue mechanical properties of high-strength aluminium processed by L-PBF

Details

Supervisors

Simar, Aude

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

High-strength aluminium alloys are widespread in the aerospace industry because of their high strength-to-weight ratio and good fatigue properties. However, the unique thermo-mechanical attributes of the Laser Powder Bed Fusion process raise several challenges due to their tendency to undergo hot cracking during solidification. A possible approach is the addition of grain refining elements to obtain crack-free parts. In this work, two different processes are used to incorporate these elements into the powder namely mechanical mixing and cold plasma coating. The optimisation and mechanical behaviour of a new zirconium-enhanced 7075 aluminium alloy powder produced by an industrialisable process, the cold plasma coating, are investigated in this work. A relative density of 99.3% and the presence of a few lack-of-fusion defects are achieved using this powder. For the optimised heat treatment, the ductility achieved is only 2.6%. The mechanical properties are attributed to the high proportion of fine grains present in the microstructure. Consequently, the process requires precise control of the incorporation of useful fine-grafted zirconium particles into the powder. Currently, studies on the influence of the building orientation and fatigue properties are scarce in literature. Tensile specimens exhibit a relative isotropic strength behaviour in as-built and T6 condition with a classical solution heat treatment temperature for wrought 7075 aluminium alloy of 470°C. However, samples with an optimised heat-treatment temperature display weaker mechanical properties in the vertical orientation. The anisotropic behaviour is more pronounced in fatigue crack growth, with a clear difference in crack propagation rate and number of cycles before fracture depending on the crack orientation. T6-aged samples exhibit a better crack growth resistance compared to wrought counterparts at low stress intensity factor range. This study investigates the resulting damage mechanisms in order to improve our understanding of the mechanical properties assessed. This analysis shows the detrimental influence of these zirconium inclusions and microstructural features such as grain boundaries present in the as-built condition.